That results from the unusual temperature dependence of coercivity in ceramic (ferrite) magnets. Coercivity is a measure of the ability of a magnet to resist demagnetization, and in most permanent magnets, coercivity decreases with increasing temperature. The coercivity of ceramic magnets instead goes through a maximum around 200 or 300 degrees centigrade, and therefore decreases as temperature is reduced below room temperature.

Ceramic magnets are often magnetized across the thin direction, perpendicular to the wide face. The magnetic poles on those wide faces create a demagnetizing field across the thickness, and as the coercivity decreases with decreasing temperature, the magnet becomes less able to resist demagnetization.